High-speed textile yarn winders conventionally employ a surface drive roll to drive a rotating tube or similar package core receiving yarn arriving at a predetermined linear speed. Such surface drive rolls are normally driven at a suitable constant rotational speed by synchronous motors. One such winder is, for example, disclosed in Schippers et al U.S. Pat. No. 3,861,607. The disclosure made in such patent is, to any extent deemed necessary for a full understanding of the present invention, hereby incorporated by reference into this specification.
Synchronous motor speed is in proportion to the frequency of the supplied alternating current and is as constant as such frequency. Normally, three phase alternating current is supplied to such motors by a frequency converter either rotary or solid state, typically consisting of an AC to DC converter operating from power line frequency, coupled to a DC to AC inverter connectable to the motor. Output frequency and motor speed have conventionally been adjustable by supplying a control signal to one of the intermediate DC stages.
For accelerating synchronous motor drives from standstill to rated synchronous running speed, its rotor has an auxiliary induction-type start-up winding. By supplying rated frequency for start-up, the motor will draw current up to twenty times rated full-load current thereby requiring that both the frequency converter and the motor be designed and built to handle currents much larger than needed to drive a high-speed winder. This approach has been expensive and, as winder speeds have increased, the expense has become prohibitive.
Several solutions to this problem have been suggested, wherein each motor or group of motors are provided with two frequency converters. In an example of this approach, on converter accelerates the motor asynchronously from standstill to a certain speed, where synchronization is effected. The frequency of this converter is than raised up to the rated frequency for driving the motor at the operational speed. The other converter then takes over to drive the motor for the winding operation. This approach has the disadvantage that the first converter and the motor must still be oversized in order to accommodate the larger than rated current demanded by the motor from the converter during mechanical synchronization at any given speed. Thus, disproportionate expense is only partially avoided, for the expense of the second converter must be added to that of oversizing.